dpi vs ppi

Resolution ppi & dpi terminology – sizing & resizing for printing – another of the confusing issues tossed about for discussion in our Grafx Group. Here I’m jotting down my understanding of the difference.

dpi (dots per inch) to me originates from the old days & is purely a printing thing … dots being the droplets of ink a printer lays down for each inch in a print. So as far as digital imaging dpi means absolutely naught. Generally the higher the dpi of the printer the better the quality print – the tonality & colours will be better & the blends between the colours will be smoother. There are many printers out there with varying dpi’s ranging from basic photo to super fine … your choice is obviously based on the size prints you are creating, for general photographic needs round 1440dpi seems to be a good choice. Can I suggest here that if you’re unfamiliar with printing & are thinking of purchasing a photo printer it would do you no harm to carry out some in depth research … there are so many photo printers out there varying in price & quality.

ppi (Pixels per inch) Basically this is the term used when talking about the size & quality of the digital image. Our image is X pixels by Y pixels … ppi represents the number of these pixels the printer will use to print one inch on paper. The higher the ppi the better the quality print we can achieve. Too few ppi in an image means that the pixels will be very large resulting in a pixelated image – an image where you can see the actual pixels giving jagged edges.

In our image editing program we have an image dialogue box – I’ll talk about PS here as that’s the program I’m most familiar with. Within this dialogue box there is a ‘resample’ check box which plays an integral role in the whole process of sizing our image … it is a very important check box! It is the one & only action that will affect the quality of information available to you from your file! Using this box you choose the print size – & the file size – by either resampling or not resampling your image.

I’ll try to explain the differences between ‘checking’ & ‘unchecking’ this box.

When resizing the image with this box checked you are, as it says, resampling the image … you are changing the pixel dimensions – not the document dimensions – of your image by either throwing away pixels or adding pixels. Setting the ppi to a smaller number you will throw away pixels setting it to a higher number you are adding pixels. Losing, or throwing away, pixels is OK provided you don’t need the bigger file again (use a copy of the original large file!) Creating, or adding, pixels is not good – you will see that the resulting quality has been degraded as these ‘new’ pixels are generated by the computer. However leaving this box unchecked, i.e. not resampling the image, you are only changing the document size not the number of pixels in our image. You are choosing how many pixels in your image you wish to make available to print one inch. This is what you would do to achieve the best quality print for your image. Again the larger this number the smaller the print & the smaller the number the larger the print. If you are outsourcing your image the default standard is 300ppi … this means that for every inch there is 300 pixels from the source image. So with this in mind – if you want a print of a determined size you must make sure the image has enough pixels to make a good quality print!

Let’s look at an example –
My camera gives me images with pixel dimensions 5616 x 3744 px.

If I wish to make a print of say 12in x 8in the ppi to print this size will be 468. This figure is calculated from the pixel dimension divided by the size of the print you want – for example using my image dimension 5616 / 12 = 468 (or 3744 / 8 = 468). From that I can see that the ppi available to make a print of this size is 468 pixels. 468 pixels are used for each inch of my print. Let PS do the calculations – in the image size dialogue box uncheck the resample image box & in the document size section select a unit & size to be printed … the resulting ppi is displayed in the resolution box.

The printer driver will now do it’s stuff translating those 468 pixels into dots per inch & produce a high quality print!

Looking at it another way … this time I want to know the maximum size I can achieve for a good quality print from my image. I know that on my printer 240ppi will be sufficient to get a good quality image – so long as my image is good & sharp!  This time the result is calculated by dividing the pixels by the ppi – in my image 5616 / 240 = 23.4in : 3744 / 240 = 15.6in.  So again let PS do the calculations – in the image dialogue box & with the resample check box unchecked I type 240 in the resolution box (ppi box) & PS calculates the ultimate size for print.

My final image will be 23.4in X 15.6in with 240 pixels used to represent each inch.

It’s important to understand the results of resampling or not resampling the image. The only reason I can think of for resampling an image to reduce its size would be to use it online or send via email.

This is my very basic understanding of the whole subject & I hope I have got it down logically! But hey don’t take my word as it … use this as a base for some further research!

Try it open an image go to > Image > Image Size & play with different combinations of controls & check boxes note down the differences. (Remember to experiment on a duplicate of the image just in case!)

Annette :)